Ion implanters have been used for many years in the processing of semiconductor wafers. Typically, a beam of ions of a required species is produced and directed at a wafer or other semiconductor substrate, so that ions become implanted under the surface of the wafer. Implantation is typically used for producing regions in the semiconductor wafer of altered conductivity state, by implanting, in the wafer, ions of a required dopant. Typical ionic species used for this purpose are boron, phosphorus, arsenic and antimony. However, other ionic species are also used for other purposes, including oxygen for example.
The depth to which implanted ions penetrate the surface of the wafer is largely dependent on the energy of the ions in the ion beam. The semiconductor industry requires both very shallow implants, for example for very fine structures having a small feature size, and relatively deep implants, for example for buried layers etc. It is also a general requirement of the semiconductor processing industry that process times should be as short as possible which implies that the quantity of ions being implanted per unit area and per unit time into a semiconductor wafer should be as high as possible. This implies that ion implantation is conducted with a high beam current, being a measure of the number of required ions in the beam reaching the wafer surface per unit time. There is also the requirement that implantation should be cost effective.
Beam energies up to about 200 keV (for singly charged ions) can quite readily be obtained using electrostatic acceleration systems, in which the source of ions is held at a fixed voltage relative to the wafer to be implanted, the fixed voltage defining the energy of the ions in the beam on implantation.
In most ion beam type ion implanters, a mass selection stage is required to select from the beam from the ion source those ionic species required for implantation. Typically mass selection is performed using a mass analysing sector magnet combined with a mass resolving slit downstream of the magnet. It is common practice in implanters using electrostatic acceleration systems for the full beam energy to be delivered to the ions of the beam prior to entering the mass analyser. However, post mass analysis electrostatic acceleration and deceleration are known, using additional electrostatically biased electrodes between the mass resolving slit and the substrate. Examples include U.S. Pat. No. 5,389,793 and U.S. Pat. No. 5,969,366.
For higher implant energies radio frequency acceleration systems have been employed, usually post mass analysis. Examples include U.S. Pat. No. 6,423,976 and U.S. Pat. No. 4,667,111 describing the use of r.f. linear accelerators, and U.S. Pat. No. 5,301,488 describing the use of r.f. quadrupole accelerator.
It is a known practice to operate ion implanters having post mass analysis accelerators (or decelerators), without energising the accelerators (or decelerators), in so-called drift mode. This practice allows the implanter to operate at lower energies (or higher for post decelerators), using the beam energy directly from the mass analyser. U.S. Pat. No. 6,423,976 describes drift mode operation of a r.f. linear type accelerator. However, the beam current available for implanting when operating in drift mode can be disappointing.